Speedbird,
If you don't get your IR within 3 years, you will have to re-take exams. I think you can just re-take the IR exams, you don't have to re-do the complete set, although I'm not 100% sure on that.
The 1500 hours is completely un-related, though - that's the requirement for the issue of a full ATPL, it's got nothing to do with getting an IR.
I don't know the details of Oxford's course, but the exams follow the same format regardless of which school you study at. They vary in length, from very short ones like the R/T exams, to the longest ones which last several hours. The number of exams each day varies throughout the week, but the number of exams each candidate sits will depend on how the schools split their modules. If you plan on being busy all day, Monday-Thursday of the exam week, you won't go wrong, although Oxford might be able to give you more accurate details of their specific module break-down.
FFF
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